1. Field of the Invention:
The present invention relates generally to the field of measuring and testing and, more specifically, to a cryogenic liquid level measuring apparatus.
2. Background of the Related Art:
Cryogenic liquids are liquified gases such as helium, hydrogen, nitrogen, oxygen, and liquid material gas, which are cooled to temperatures below 120K. These gases have industrial importance and will become more important as alternative fuels for vehicles, since some of these must be kept at cryogenic temperatures so that they do not occupy as much volume as the gases.
Liquid helium is currently being used to keep superconductors cool and thus, as the critical temperature of superconductor materials continue to increase, and thus as the superconductors become more and more practical, there will be an increasing demand for cryogenic liquids. Superconductors are used in making magnets for bending electron beams, for example, in continuous electron beams for probing the atom, and also for superconducting cavities.
At present, techniques used for measuring cryogenic liquid levels include the use of superconducting wires wherein part of the wire that is in the liquid has no resistance but the wire above the liquid does have a resistance and the level of liquid is determined by the amount of resistance in the total wire. The resistance is correlated to a level of liquid.
Another technique is to measure capacitance, between two concentric cylinders (tubes) the capacitance of the space between the concentric cylinders of the immersed part is different than the concentric cylinders in the gaseous part. This difference is used to determine the liquid level.
The normal operation of a superconducting wire liquid helium level sensor depends on the difference in heat transfer between liquid and vapor of helium. Reliable operation of the level sensor and a wide operating range (1.5 to 5K) relies on the critical heat flux of saturated pool boiling on the superconducting wire of the sensor. The critical heat flux of the level sensor is related to the operating temperature and current of the sensor.
FIGS. 1(a)-1(c) the voltage-current characteristic of the sensor at different temperatures. The hysteresis curve "C" is normal for a properly working sensor. The characteristic consist of non-linear (I), linear (II) and non-linear (III) segments. The first non-linear segment is related to the growth of the normal zone of the superconductor with increasing current towards the liquid-vapor interface. The linear segment is due to the linear dependence of the fixed normal zone above the liquid-vapor interface with increasing current. The second non-linear part is due to the driving down of the superconductor-normal interface inside the liquid because of the onset of film boiling. The current corresponding to the onset of the non-linear segment III can be termed as the critical heater current at which film boiling initiates.
FIG. 2 illustrates the critical heater current as a function of liquid helium bath pressure. For a comparison, FIG. 3 shows the critical heat flux versus pressure for horizontal wires. The qualitative agreement between the figures is evident. (FIG. 3 is taken from "Effects of Diameter and System Pressure on Critical Heat Flux for Horizontal Cylinders and Saturated Liquid Helium" published in Cryogenics, June 1989, Vol. 29).)
These level sensors are normally operated with 70 mA current and a line has been drawn to indicate this in FIG. 2, from which the following conclusions can be drawn. The sensor will not operate at temperatures above 4.6K, and the sensor will either indicate lower level or even hang up in the temperature range of 2.18K to 3.0K.
One could lower the sensor current to try to reliably operate the sensor in the above temperature ranges. However, this may reduce the sensor response time or even make it unstable depending upon the helium flow conditions.
Thus, a need exists for a new level sensor which is not subjected to the aforementioned drawbacks associated with known devices.